1. Field of the Invention
The present invention relates to an X-Y address type solid state image sensing device represented by a CMOS type image sensing device and an amplification type image sensing device and an image signal processing method of the same, and particularly to a current output type solid state image sensing device and a processing method for processing an image signal of the same.
2. Description of Related Art
A structure of a related art of a solid state image sensing device, for example, a CMOS type image sensing device which outputs an image signal as a current is illustrated in FIG. 8.
In FIG. 8, a unit pixel 101 is formed of a photo-diode 102, a transistor 103 for amplification, a transistor 104 for vertical selection and a transistor 105 for reset. This unit pixel 101 is arranged in the X direction (column direction) and Y direction (row direction). Here, in order to simplify the figure, only the pixels of m-row and n-column are illustrated.
In this unit pixel 101, a vertical scanning pulse xcfx86 Vm is impressed to the gate electrode of the transistor 104 for vertical selection via a vertical selection line 107 from a vertical scanning circuit 106 and a vertical reset pulse xcfx86 VRm is impressed to the gate electrode of the transistor 105 for reset via a vertical reset line 108 from the vertical scanning circuit 106. Moreover, signal charge photo-electrically converted by a photo-diode 102 is then converted to a signal current by the transistor 103 for amplification and is then output to a vertical signal line 109 via the transistor 104 for vertical selection.
Between the vertical signal line 109 and horizontal signal line 110, a transistor 111 for horizontal selection is connected. To the gate electrode of the transistor 111 for horizontal selection, a horizontal scanning pulse xcfx86 Hn is impressed to a horizontal scanning circuit 112. Thereby, a signal current output to the vertical signal line 109 from the pixel 101 flows into the horizontal signal line 110 through the transistor 111 for horizontal selection.
At the end part of the horizontal signal line 110, a current/voltage converting circuit 113 is provided. This current/voltage converting circuit 113 is composed of a differential amplifier 114 connected at its inverted (xe2x88x92) input terminal to the horizontal signal line 110 and a feedback resistor 115 connected between the inverted input terminal and an output terminal of the differential amplifier 114. Moreover, to the non-inverted (+) input terminal of the differential amplifier 114, a bias voltage Vbias is applied and a signal current input through the horizontal signal line 110 is converted to a signal voltage and is then output.
In a MOS type image sensing device of the related art of the structure explained above, when the on-chip structure is realized by fabricating a current/voltage converting circuit 113 on the same semiconductor substrate as pixel portion, a power source voltage VDD of the current/voltage converting circuit 113 usually becomes single power source voltage such as 5 V or 3 V. Therefore, when the circuit configuration formed of the differential amplifier 114 and feedback resistor 115 as explained above is used as the current/voltage converting circuit 113, this circuit assures sufficient output voltage range only by setting the bias voltage Vbias to almost a half of the power source voltage VDD.
As explained above, when the bias voltage Vbias of the current/voltage converting circuit 113 is set to about a half of the power source voltage VDD, each potential of the vertical signal line 109 and horizontal signal line 110 to which a signal current is output from the pixel 101 becomes almost equal to the bias voltage Vbias, namely to about a half of the power source voltage VDD. Therefore, only a voltage (about a half of the power source voltage VDD) equal to the potential difference between the power source VDD and signal lines 109, 110 is applied across the drain and source of the transistor 103 for amplification in the pixel 101. Here, voltage drop of the transistor 104 for vertical selection and transistor 111 for horizontal selection is neglected here for discussion.
Therefore, when the power source voltage VDD is set, for example, to 3.0 V, only a voltage of 1.5 V is applied across the drain and source of the transistor 103 for amplification. Accordingly, the transistor 103 for amplification operates under the condition that its mutual conductance gm is rather lower. As a result, signal voltage of the photo-diode 102 cannot be converted to a signal current with a sufficient amplification factor and as a result, sensitivity of image sensing device is lowered.
Moreover, even in the same related art, it has also been proposed that each potential of the vertical signal line 109 and horizontal signal line 110 is lowered so that a higher voltage is applied across the drain and source of the transistor 103 for amplification in the unit pixel 101 in order to improve the sensitivity. For this purpose, the bias voltage Vbias of the current/voltage converting circuit 113 is lowered and the part, in which the input/output characteristic is not so good, of the differential amplifier 114 is used.
Drop of the bias voltage Vbias of the current/voltage converting circuit 113 corresponds to change of IN+=xc2xd*VDD to IN+=xc2xc*VDD in the input/output characteristic of the differential amplifier illustrated in FIG. 4. The dotted line of INxe2x88x92=OUT written over the input/output characteristic indicates the operating point when the signal current is zero in the current/voltage converting circuit 113 and a thick solid line enclosed in the circle indicates the input/output characteristic (Rout) up to saturation from zero of the pixel signal current under the condition of IN+=xc2xc*VDD.
Here, when attention is paid to the part of input/output characteristic enclosed by a circle, since the input/output characteristic is not indicated by a linear line, it can be understood that linearity is deteriorated and gain is low. Namely, this input/output characteristic indicates that as a result of compulsory adjustment of the bias voltage Vbias of the current/voltage converting circuit 113 in order to improve sensitivity of pixel, linearity and gain of the current/voltage converting circuit 113 are deteriorated.
The present invention is proposed considering the background explained above and it is therefore an object of the present invention to provide a solid state image sensing device which can improve amplification factor (sensitivity) of pixel while linearity of the current/voltage converting circuit is maintained and also provide a pixel signal processing method of the same.
In order to attain the object explained above, in the solid state image sensing device of the present invention in the structure that an image signal of each pixel is output to a signal line as the current and a signal current output to the signal line is converted to a signal voltage and it is then output, an input operation point of a current voltage converting means for converting a signal current to a signal voltage is set approximate to the ground level or power source level and a pixel signal can be derived by converting the signal current to the signal voltage with this current voltage converting means.
In a current output type solid state image sensing device, the potential of signal line can be lowered (or raised) by setting the input operation point of the current voltage converting means approximate to the ground level (or power source level). Thereby, a higher voltage can be applied to the transistor for amplification of the pixel. Sensitivity (amplification factor) of pixel can be raised by applying a higher voltage to the transistor for amplification.
Here, it is particularly preferable that the present invention is adapted to a CMOS type image sensing device where a current voltage circuit is formed on the same semiconductor substrate chip as the pixel part.